DE 10 2005 019 515 A1 describes a method for measuring the speed of an electronically commutated motor, which has a primary part with a winding and a secondary part with magnet segments which are offset with respect to one another in the circumferential direction and are magnetized alternately in mutually opposite directions, wherein the secondary part is rotated relative to the primary part. A plurality of magnetic field sensors detect the position of the magnet segments, wherein the time between two changes in the position measurement signal is measured. During the calculation of the motor speed, the influence of tolerances of the magnet segments in terms of their positioning and/or dimensions is compensated for, wherein in each case one correction factor is determined, stored and used for the correction of the speed signal, preferably for each of the magnet segment/sensor combinations.
The above-described method in accordance with the prior art has the disadvantage that errors caused by different switching thresholds of the different magnetic field sensors can only be corrected when in each case one correction factor is determined for each magnet segment/sensor combination for both directions of rotation of the motor. After deactivation of the motor, for example during a standstill of a vehicle having the motor, a complete set of correction factors needs to be determined again and adjusted with the stored values, which is very time-consuming owing to the multiplicity of correction factors required. A dependence of the correction factors on the speed of the motor can also occur, since said correction factors cannot simultaneously correctly describe the influence of angular errors and signal propagation times.
Therefore, it is the object of the present invention to provide a method for determining the motor speed which does not have the above mentioned disadvantages.
This object is achieved according to this invention.